The idea that space can bend is a result of the application of theoretical physics. It is based on arguments of if hypothesis A is true, then hypothesis B must be true. “Hypothesis A” refers to the geodesic behavior of light, in other words light always travels the shortest distance from one point to another. “Hypothesis B” is the conclusion that gravity bends space. However, Hypothesis A seems to be contradicted by an important observation of the effect that a big mass in space has on light.
Four galaxies have been observed to be tightly spaced out evenly around a circle by a deep space telescope. When these galaxies were examined closely it was found that they looked exactly the same. But it is so unlikely to find four galaxies that look exactly the same so close together that it can be said that the galaxies are not four different galaxies but rather the same galaxy four times. This conclusion is not only drawn from the fact that they look the same, but it is also extremely strange that, if they were four different galaxies, the gravitational force between them hasn’t caused the four galaxies to crash into each other. A reasonable explanation would be that there is something, very likely a large mass like a black hole, between us, the observer, and the galaxy which has some effect on the light that travels past it.
Light behaves the most strangely of anything else in everyday life. It has been mentioned that light is bend around large masses. However, many experiments have concluded that light is not affected by gravity. So, how can light be bent by a large mass if not by gravity? Gravity, as well as acting as a force between any two masses, has an effect on space itself. In fact it is said to bend space. It has been stated that the reason that gravity acts as an attracting force between two masses is because it bends space, which can be understood as that a plane (one two dimensional part of the space we live in) is affected by a mass as tough a large stone has been placed on a piece of cloth. If a marble is placed near the stone on the cloth its natural tendency is to roll towards the large stone which is similar to the natural tendency of objects to fall towards the Earth.
An important property of light is that it follows a geodesic path from one point to another, which means that it travels the shortest distance between two points. It is also said that the speed of light is the fastest speed any object can travel. This comes from observations that the measured speed of light in a vacuum is constant no matter the setting (even if the speed of light is measured in a spaceship that travels at the speed of light, however, if an outside observer was to look inside the shuttle they would observe the light to be travelling at the speed of light). These two statements can be combined to state that it is impossible for anything to reach a point faster than light can if they both start at the same point (for instance the object competing with light starts at the source of that light).
Here a contradiction between two statements occurs. The first observation was that light bends around large masses. The second statement would contradict this as light follows a geodesic path. So, theoretically, it would be possible to get from one point in space to another before the light would arrive (for instance a light source is placed at some distance away from a large mass and a space shuttle that travels near the speed of light starts to travel in a straight line towards a target at which the path of the light meets the path of the space shuttle. If the mass is large enough the space shuttle could arrive at the target faster than the light since its path is shorter than the path of the light), as it is being bent by the large mass which means that it follows a path that is longer than the direct path. But previously we have established that this is impossible. The statement of light following a geodesic path through space is a reoccurring statement because it is the critical step towards understanding that space itself has to bend so that our observations here on Earth fit observations that we make in other parts of the universe. If we say that space itself bends light still travels the shortest possible distance between two points, its path is still geodesic. If this thought is continued a question might arise: What if the mass is very large?
There are several real settings where a mass is very large such as a black hole. It is said that the gravitational force is so strong that the escape velocity from a black hole is larger than the speed of light, therefore light cannot escape from a black hole. But light is not affected by gravity, so how can this statement be true? For this we can go back to an earlier statement, that gravity bends space. It can be concluded that the intensity of the gravity bends space “infinitely”, in other words space curves back on itself. Thus we can say that light cannot escape a black hole because space is bent to such a degree that light does not go anywhere. However, light still travels at the speed of light. This can be understood as that the direction of the speed of the light is constantly reversed, so it is still travelling at the speed of light but it does not go anywhere.
Light is not affected by gravity, which is a conclusion that has been drawn from experiments, but it has been observed that large masses can have an effect on light. Since light always follows the geodesic path between two points it can be concluded and proven that gravity bends space.
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